1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for trailing edge dimmer compatibility with dimmer high resistance prediction.
2. Description of the Related Art
The development and use of energy efficient technologies continues to be a high priority for many entities including many companies and countries. One area of interest is the replacement of incandescent lamps with more energy efficient lamps such as lamps based on electronic light sources. For this description, electronic light sources are light emitting diodes (LEDs) and compact fluorescent lamps (CFLs). The development of electronic light source based lamps and are not without many challenges. One of the challenges is developing electronic light source based lamps that are compatible with existing infrastructure. The following discussion focuses on LED-based lighting systems but is also applicable to CFL-based lighting systems and combination LED and CFL based lighting systems.
Many electronic systems include circuits, such as switching power converters that interface with a dimmer. The interfacing circuits deliver power to a load in accordance with the dimming level set by the dimmer. For example, in a lighting system, dimmers provide an input signal to a lighting system. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers generate a digital or analog coded dimming signal that indicates a desired dimming level. A trailing edge dimmer phase cuts a trailing edge of an alternating current (“AC”) supply voltage.
FIG. 1 depicts a lighting system 100 that includes a trailing edge, phase-cut dimmer 102. FIG. 2 depicts an exemplary, trailing edge phase cut voltage graph 200 and a dimmer control signal 201 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VIN from voltage supply 104. The supply voltage VIN, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. The trailing edge dimmer 102 phase cuts trailing edges, such as trailing edges 202 and 204, of each half cycle of supply voltage VIN. Since each half cycle of supply voltage VIN is 180 degrees of the supply voltage VIN, the trailing edge dimmer 102 phase cuts the supply voltage VIN at an angle greater than 0 degrees and less than 180 degrees. The phase cut, input voltage VΦ—IN to the lighting system 100 represents a dimming level that causes the lighting system 100 to adjust power delivered to a lamp 106, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp 106. The lamp 106 is an incandescent lamp and can generally be modeled as a resistor 108.
The dimmer 102 includes a timer controller 110 that generates dimmer control signal DCS to control a duty cycle of switch 112. The duty cycle of switch 112 is a pulse width, e.g. times t1-t0, divided by a period of the dimmer control signal, e.g. times t3-t0, for each cycle of the dimmer control signal DCS. The timer controller 110 converts a desired dimming level into the duty cycle for switch 112. The duty cycle of the dimmer control signal DCS is decreased for lower dimming levels, i.e. higher brightness for lamp 106, and increased for higher dimming levels. During a pulse, e.g. pulse 206 and pulse 208, of the dimmer control signal DCS, the switch 112 conducts, i.e. is ON, and the dimmer 102 enters a low resistance state. In the low resistance state of the dimmer 102, the resistance of the switch 112 is, for example, less than or equal to 10 ohms. During the low resistance state of switch 112, the phase cut, input voltage VΦ—IN tracks the input supply voltage VIN and the dimmer 102 transfers a dimmer current iDIM to the lamp 106.
When the timer controller 110 causes the pulse of the dimmer control signal 206 to end, the dimmer control signal 206 turns the switch 112 OFF, which causes the dimmer 102 to enter a high resistance state, i.e. turns OFF. In the high resistance state of the dimmer 102, the resistance of the switch 112 is, for example, greater than 1 kohm. The dimmer 102 includes a capacitor 114, which charges to the supply voltage VIN during each pulse of the timer control signal DCS. In both the high and low resistance states of the dimmer 102, the capacitor 114 remains connected across the switch 112. When the switch 112 is OFF and the dimmer 102 enters the high resistance state, the voltage VC across capacitor 114 decays, e.g. between times t1 and t2 and between times t4 and t5. The rate of decay is a function of the amount of capacitance C of capacitor 114 and the dimmer current iDIM that is transferred through the resistance R of lamp 108. Equation [1] represents the relationship between the capacitance C of capacitor 114, the dimmer current iDIM, and the rate of decay dVΦ—IN of the phase cut, input voltage VΦ—IN:iDIM×C·dV101—IN/dt  [1]
The resistance value R of lamp 106 is relatively low and permits a high enough value of the dimmer current iDIM to allow the phase cut, input voltage VΦ—IN to decay to a zero crossing, e.g. at times t2 and t5, before the next pulse of the dimmer control signal DCS.
Trailing edge dimmers, such as trailing edge dimmer 102, have some favorable characteristics. For example, trailing edge dimmer 102 does not have an abrupt voltage increase when the dimmer 102 begins to conduct, e.g. at times t0 and t3, and has a decaying decrease when the dimmer 102 enters the high resistance state. Thus, harmonic frequencies are lower, and the dimmer 102 generates less electromagnetic interference.
As previously discussed, electronic light sources have a higher energy efficiency than incandescent lamps of comparable light out. Thus, electronic light sources are being retrofitted into existing infrastructure that includes trailing edge dimmers, such as trailing edge dimmer 102. An electronic light source has lower power requirements and, thus, less dimmer current iDIM is transferred to the electronic light sources. Thus, in accordance with Equation [1] for a smaller dimmer current iDIM, the decay rate dVΦ—IN/dt is less. If the decay rate dVΦ—IN/dt is too low, the phase cut, input voltage VΦ—IN does not reach a zero crossing prior to a beginning of a next cycle of the supply voltage VIN. Failure to reach a zero-crossing can cause some trailing edge dimmers to malfunction.
FIG. 3 depicts a lighting system 300 that includes the trailing edge dimmer 102 and LED(s) 302. The dimmer 102 functions as previously described and provides a phase cut, input voltage VΦ—IN and a dimmer current to a full bridge diode rectifier 304. The rectifier 304 provides the phase cut, rectified voltage VΦ—R to a power converter 306. The power converter 306 respectively converts the phase cut, rectified voltage VΦ—R and the rectified input current iR into an approximately constant output voltage VOUT and an output current iOUT. The output current iOUT adjusts with the dimming level indicated by the phase angle of the phase cut, input voltage VΦ—IN and is approximately constant for any given dimming level.
The controller 308 includes a current controller 310 to control the transfer of current iR to the power converter 306 and regulate the power delivered to the LED(s) 302. The LED(s) require substantially less power to provide the equivalent light output of an incandescent bulb. For example, the LED(s) 302 use 4 W of power to provide the equivalent light output of a 60 W incandescent bulb. The output voltage VOUT is generally boosted by the power converter 306 to, for example, 400V. Since the power P provided to the LED(s) 302 is approximately P=VOUT·iOUT, a maximum current iR transferred to the power converter 306 is typically only 50 mA, which is less than the approximately 545 mA maximum current drawn by a 60 W bulb from a 110 V supply input voltage VIN. Thus, the decay time dVΦ—IN/dt for the lighting system 300 increases in accordance with Equation [1]. The controller 308 includes a comparator 312 to detect trailing edges, such as trailing edges 314 and 316, of the phase cut, rectified voltage VΦ—R.
Detection of the trailing edge of the phase cut, rectified voltage VΦ—R is not a simple task. The trailing edges of rectified input voltage VΦR—IN at times t1 and t4 are generally noisy and may contain other distortions. To detect the trailing edges, the controller 308 utilizes a comparator 312 to detect the trailing edge at a more stable portion of the phase cut, rectified voltage VΦ—R. The comparator 312 receives the phase cut, rectified voltage VΦ—R or a scaled version of the phase cut, rectified voltage VΦ—R at an inverting input of the comparator 312. The comparator 312 compares the phase cut, rectified voltage VΦ—R with a fixed, trailing edge detection voltage threshold, such as +20V, and generates a trailing edge detection signal TE_DETECT. The trailing edge detection signal TE_DETECT signal is a logical 0 prior to detection of a trailing edge of phase cut, rectified voltage VΦ—R and transitions to a logical 1 upon detection of the trailing edge. Once the trailing edge detection signal TE_DETECT indicates detection of the trailing edge, the current controller 310 increases a transfer of current iDIM through the dimmer 102 to increase the rate of decay dVΦ—IN/dt and, thus, increase the rate of decay of the phase cut, rectified voltage VΦ—R at, for example times t2 and t4. Increasing the rate of decay at times t2 and t5 helps ensure that the phase cut, rectified voltage VΦ—R reaches a zero crossing prior to a beginning of a next cycle of the phase cut, rectified voltage VΦ—R. The trailing edge detection threshold value is set low enough to avoid prematurely detecting a trailing edge. However, because the rate of decay dVΦ—IN/dt is greater for electronic light sources, the low value of the trailing edge detection threshold also means that the trailing edge might not be detected before a zero crossing of the phase cut, rectified voltage VΦ—R for large phase angles. Increasing the value of the trailing edge detection threshold can result in transferring an unnecessary amount of current from the voltage supply 104.
It is desirable to improve compatibility with trailing edge dimmers.